Radioactive process for detection of surface contamination



United States Patent 3,215,839 RADIOACTIVE PROCESS FGR DETECTION OF SURFACE CONTAMINATION John Lynde Anderson, Orlando, Fla., assignor to Cleanometer Corporation, Wilmington, Del. No Drawing. Filed Dec. 27, 1960, Ser. No. 78,282 8 Claims. (Cl. 250-106) This invention relates to a new method of detecting contamination on surfaces and more particularly to the detection of non-volatile organic contamination on metal surfaces.

The rapid and quantitative detection of non-volatile organic contamination either on metal surfaces or within spaces confined by metal surfaces, particularly contaminants which are present only in very small amounts, is not practicable using existing methods and techniques. Thus a method in general use involves the process of thoroughly wetting a metal surface with a solvent for the contaminant, of recovering the solvent quantitatively and then of determining the amount of contaminant dissolved in the solvent. This method, however, determines only material which is removed from the surface and does not determine material which is left on, or is adjacent to, the surface after the test is completed. The problem of contamination .of metal surfaces often is a most serious one, particularly when the contamination may cause explosions or rapid, uncontrolled chemical reactions when in contact with other material for which the surface in its various forms is designed. For example, in handling liquid oxygen it is essential that the amount of organic contamination on the metal surfaces over which the liquid oxygen is standing or through which it is transferred must be kept to the irreducible minimum and must be quantitatively determined prior to permitting the liquid oxygen to come into contact with the metal surfaces involved, in order to insure maximum stability and safety.

It is therefore an object of this invention to provide a new and useful method for the detection of contamination on surfaces. Another object is the provision of a method for the detection of non-volatile organic contamination on metal surfaces or within spaces confined by metal surfaces by a simple and economical method. A still further object is the provision of a method by which non-volatile hydrocarbon contamination on metal surfaces may be detected. Other objects will appear as the description of the invention proceeds.

These and other objects are accomplished in the present invention by applying a non-volatile radioactive compound dissolved in a volatile solvent to a surface, by

evaporating the volatile solvent, and by detecting the amount of radioactivity which is left on the surface being examined, the amount of radioactivity which is determinable after such evaporation of the solvent on a previously scrupulously clean surface similarly treated being more than that which is determinable on a surface which is contaminated with a non-volatile contaminant miscible with the radioactive compound.

More particularly my invention includes the process of applying a non-volatile radioactive labeled compound, chemically compatible with the organic materials normally found on metal surfaces, to a metal surface, of evaporating the volatile solvent used to apply the labeled compound, and of detecting the amount of radioactivity which remains on the surface being examined, the amount of radioactivity determinable after the evaporation of the solvent being less when the surface examined originally bore a non-volatile organic hydrocarbon contaminant than when it was scrupulously clean prior to the application of the labeled compound.

Patented Nov. 2, 1965 In my invention a non-volatile radioactive labeled com pound is used which is compatible and miscible with the type of contamination to be detected and to be measured and it is necessary that the radioactive labeled compound be chemically inert with both the surface and the contaminant. In the event that the radioactive labeled compound is, in fact, somewhat reactive with either the surface or the contaminant, my inventive process still is valid provided that a similar degree of reactivity exists or that no significant amounts of radioactivity are lost during the evaporative phase. Within rather broad limits, the amount of radioactivity which is detectable is, in general, inversely proportional to the amount of contamination originally present. The physical state and the chemical nature of the contaminant also alfect to some degree the amount of residual activity detected.

My inventive process is based on the principle of isotopic dilution and the phenomenon associated with absorption of radiation by chemical compounds. In general only very small quantities of the particular radioactive labeled compound are added to a surface being examined so that approximately ten times or more of contamination may be encountered. The resultant chemical dilution causes a similar-fold dilution of the specific activity of the radiochemical and measurement of the radiation which arises from the surface of the mixture of contaminant and radiochemical is less than that which arises from pure radiochemical when similar areas and concentrations of original radiochemical are used.

The solventselected for my process must be a solvent not only for the radiochemical but also for at least a part of the contamination so that effective chemical mixing can be achieved. The type of contamination which is best detected by my inventive process is that which is often found on metal surfaces such as the organic compounds from cutting or threading oils, from greases and protective lubricants, and from hydrocarbon fuels. Amounts of contamination down to 5 micrograms per square inch and even lower are readily detected particularly when the radiochemical has a very high specific activity such as 10 to 25 millicuries per millimole and when a microgram or less of radiochemical is applied per square inch of surface.

It is often desirable that a reference area, scrupulously cleaned, be used with substantially the same amount of applied radiochemical per unit area as is employed with the particular area being examined to insure the greatest accuracy of the detection process. When a reference area and surface is used, it is important that the amount of radioactive labeled compound per unit area applied to the reference surface be equal to that applied to the surface being tested, otherwise inconsistent results are sometimes obtained.

The particular radioactive compound employed in my inventive process is selected for compatability with the possible contaminant and for complete non-volatility under the conditions of the process. The radiochemical may emit alpha, beta, or gamma radiation although in general the relatively soft beta emissions permit convenient detection and sufficient self-absorption by the contaminants normally encountered to be preferred. Organic compounds which contain carbon-14, sulfur- 35, or hydrogen- 3 are entirely suitable for the purpose of my invention provided of course the conditions of chemical compatibility and non-volatility are also met.

The radioactivity in my inventive process is detected by means of any suitable detector of radiation given off by the particular radioisotope employed. For weak beta emitters, a thin window Geiger-Mueller tube and associated equipment is acceptable and convenient. Portable instruments capable of detecting radiation inside of long and complex metal tubes are especially valuable for determining contamination in assembled piping systems.

The following example is given by way of illustration only and not by way of limitation since the particular radioactive labeled compound, the surfaces, the solvent, the condition of the surfaces, and the amount of radiochemical applied may vary in the example within the scope of the above disclosure. It is also to be understood that while the invention is illustrated in the example by the use of a carbon-14 labeled compound materials labeled with other suitable radioactive isotopes may be used. In the example, the proportions are expressed in parts by weight unless otherwise specified.

Example I One part of a solution of 1000 parts of trichloroethylene and 1 part of n-triacontane-C14 containing 20 microcuries of radioactivity is added to one square inch of a stainless steel surface containing dispersed uniformly on the surface 20 micrograms of non-volatile hydrocarbon. An additional part of the solution is added to a scrupulously clean stainless steel surface of similar area and characteristics. The trichloroethylene solvent is evaporated completely. Determination of the radioactivity with a thin window Geiger-Mueller detector shows that the originally contaminated surface exhibits substantially less radiation than is detected similarly over the originally clean surface.

The processes of this invention are useful in many ways. They are used to detect the presence of hydrocarbon and other such contaminants on the surface of metal plates, valves, tanks and containers, pipes and tubes, and of other metal shapes. The residual hydrocarbon and other contaminants which are detected by the invention herein described are not in general directly determinable by other means. The use of the invention herein described is especially important when the surfaces so examined are used for the storage or transfer of materials which react violently with the contaminants so detected since the process permits the determination not only of the level of contamination but also the establishment of realistically acceptance levels of contamination to prevent the possibility of violent reaction.

It will be apparent that many different embodiments of this invention may be made without departing from the spirit and the scope thereof. And therefore it is not intended to be limited except as indicated in the appended claims.

I claim:

1. A process for the determination of contamination on a surface which comprises dissolving a non-volatile, radioactive compound in a volatile solvent which is a solvent for at least some part of said contamination; contacting the surface to be tested with the resultant solution to contact any of said contamination, effecting thereby an impregnation of said contamination with said radiochemical and said solvent; evaporating the said solvent; measuring the radioactivity along the surface thus treated to determine the amount of absorption and adsorption of said radiochemical along said surface; and comparing the radioactivity measurements with the radioactivity measurements on a control run effected by applying the above said steps to a like surface in an uncontaminated condition.

2. A process in accordance with claim 1 in which the contamination is organic.

3. A process in accordance with claim 2 in which the organic contamination is a hydrocarbon.

4. A process in accordance with claim 1 in which the radioactive compound is a beta emitter.

5. A process in accordance with claim 4 in which the said radioactive compound is radioactive by the presence of carbon-l4.

6. A process in accordance with claim 4 in which said radioactive compound is n-triacontane-C.

7. A process in accordance with claim 1 in which the said solvent is trichloroethylene.

8. A process for the detection of contamination on a surface which comprises dissolving n-triacontane-C in trichloroethylene; contacting the surfaces with the resultant solution; evaporating the trichloroethylene; measuring the radioactivity along the surface so treated; and comparing the results with radioactivity measurements obtained in a control run effected by applying the above steps to a like surface in an uncontaminated condition.

References Cited by the Examiner UNITED STATES PATENTS 2,367,949 1/45 Langer 25043.5 2,660,678 11/53 Sigworth et al. 250106 2,769,913 11/56 Mazzagatti 25043.5 2,933,604 4/60 Norton 250106 2,946,891 7/60 Wesolowski 250106 2,968,733 1/61 Dvorkovitz et al. 25043.5 3,014,054 12/61 Fries 250106 3,020,409 2/ 62 Clement 250-106 3,065,350 11/62 Graner 250-106 FOREIGN PATENTS 125,932 4/59 Russia.

OTHER REFERENCES Bacon, Radioactive Tracers Used in Corrosion Studies, General Electric Review, May 1949, pp. 7 to 9. Thompson, Biological Applications of Tritium, Nucleonics, September 1954, p. 31 to 35.

Fighting Flux Contamination, Electronic Industries, November 1959, pp. 110 and 246.

RALPH G. NILSON, Primary Examiner. 

1. A PROCESS FOR TH DTERMINATINOF CONTAMINATION ON A SURFACE WHICH COMPRISES DISSOLVING A NON-VOLATILE, RADIOACTIVE COMPOUND INA VOLATILE SOLVENT WHICH IS A SOLVENT FOR AT LEAST SOME PART OF SAID CONTAMINATION; CONTACTING TH SURFACE TO BE TESTED WITH THE RESULTANT SOLUTION TO CONTACT ANY OF SAID COMTAMINATION, EFFECTING THEREBY AN IMPREGNATION OF SAID CONTAMINATION WITH SAID RADIOCHEMICAL AND SAID SOLVENT; EVAPORATING THE SAID SOLVENT; MEASURING THE RADIOACTIVITY ALONG THE SURFACE THUS TREATED TO DETERMINE THE AMOUNT OF ABSORPTION AND ADSORPTION OF SAID RADIOCHEMCIAL ALONG SAID SURFACE; AND COMPARING THE RADIOACTIVITY MEASUREMENTS WITHTHE RADIOACTIVITY MEASUREMENTS ON A CONTROL RUN EFFECTED BY APPLYNG THE ABOVE SAID STEPS TO A LIKE SURFACE IN AN UNCONTAMINATED CONDITION. 